A Digital Television Standard published Sep. 16, 1995 by the Advanced Television Systems Committee (ATSC) specifies vestigial sideband (VSB) signals for transmitting digital television (DTV) signals in 6-MHz-bandwidth television channels such as those currently used in over-the-air broadcasting of National Television System Committee (NTSC) analog television signals within the United States. The VSB DTV signal is designed so its spectrum is likely to interleave with the spectrum of a co-channel interfering NTSC analog TV signal. This is done by positioning the pilot carrier and the principal amplitude-modulation sideband frequencies of the DTV signal at odd multiples of one-quarter the horizontal scan line rate of the NTSC analog TV signal that fall between the even multiples of one-quarter the horizontal scan line rate of the NTSC analog TV signal, at which even multiples most of the energy of the luminance and chrominance components of a co-channel interfering NTSC analog TV signal will fall. The video carrier of an NTSC analog TV signal is offset 1.25 MHz from the lower limit frequency of the television channel. The carrier of the DTV signal is offset from such video carrier by 59.75 times the horizontal scan line rate of the NTSC analog TV signal, to place the carrier of the DTV signal about 309,877.6 kHz from the lower limit frequency of the television channel. Accordingly, the carrier of the DTV signal is about 2,690122.4 Hz from the middle frequency of the television channel. The exact symbol rate in the Digital Television Standard is (684/286) times the 4.5 MHz sound carrier offset from video carrier in an NTSC analog TV signal. The number of symbols per horizontal scan line in an NTSC analog TV signal is 684, and 286 is the factor by which horizontal scan line rate in an NTSC analog TV signal is multiplied to obtain the 4.5 MHz sound carrier offset from video carrier in an NTSC analog TV signal. The symbol rate is 10.762238*10.sup.6 symbols per second, which can be contained in a VSB signal extending 5.381119 MHz from DTV signal carrier. That is, the VSB signal can be limited to a band extending 5.690997 MHz from the lower limit frequency of the television channel.
The ATSC standard for digital HDTV signal terrestrial broadcasting in the United States of America is capable of transmitting either of two high-definition television (HDTV) formats with 16:9 aspect ratio. One HDTV format uses 1920 samples per scan line and 1080 active horizontal scan lines per 30 Hz frame with 2:1 field interlace. The other HDTV format uses 1280 luminance samples per scan line and 720 progressively scanned scan lines of television image per 60 Hz frame. The ATSC standard also accommodates the transmission of DTV formats other than HDTV formats, such as the parallel transmission of four television signals having normal definition in comparison to an NTSC analog television signal.
DTV transmitted by vestigial-sideband (VSB) amplitude modulation (AM) during terrestrial broadcasting in the United States of America comprises a succession of consecutive-in-time data fields each containing 313 consecutive-in-time data segments. There are 832 symbols per data segment. So, with the symbol rate being 10.76 MHz, each data segment is of 77.3 microseconds duration. Each segment of data begins with a line synchronization code group of four symbols having successive values of +S, -S, -S and +S. The value +S is one level below the maximum positive data excursion, and the value -S is one level above the maximum negative data excursion. The initial line of each data field includes a field synchronization code group that codes a training signal for channel-equalization and multipath suppression procedures. The training signal is a 511-sample pseudo-random noise sequence (or "PN-sequence") followed by three 63-sample PN sequences. The middle 63-sample PN sequence is transmitted in accordance with a first logic convention in the first line of each odd-numbered data field and in accordance with a second logic convention in the first line of each even-numbered data field, the first and second logic conventions being one's complementary respective to each other. The remainder of the training signal is transmitted with same logic convention in all data fields.
The subsequent lines of each data field contain data that have been Reed-Solomon forward error-correction coded. In over-the-air broadcasting the error-correction coded data are then trellis coded using twelve interleaved trellis codes, each a 2/3 rate trellis code with one uncoded bit. Trellis coding results are parsed into three-bit groups for over-the-air transmission in eight-level one-dimensional-constellation symbol coding, which transmission is made without symbol pre-coding separate from the trellis coding procedure. Trellis coding is not used in cablecasting. The error-correction coded data are parsed into four-bit groups for transmission as sixteen-level one-dimensional-constellation symbol coding, which transmissions are made without precoding.
The VSB signals have their natural carrier wave, which would vary in amplitude depending on the percentage of modulation, suppressed. The natural carrier wave is replaced by a pilot carrier wave of fixed amplitude, which amplitude corresponds to a prescribed percentage of modulation. This pilot carrier wave of fixed amplitude is generated by introducing a direct component shift into the modulating voltage applied to the balanced modulator generating the amplitude-modulation sidebands that are supplied to the filter supplying the VSB signal as its response. If the eight levels of 4-bit symbol coding have normalized values of -7, -5, -3, -1, +1, +3, +5 and +7 in the carrier modulating signal, the pilot carrier has a normalized value of 1.25. The normalized value of +S is +5, and the normalized value of -S is -5.
VSB signals using 8-level symbol coding will be used in over-the-air broadcasting within the United States, and VSB signals using 16-level symbol coding are proposed in the ATSC standard for use in over-the-air narrowcasting systems or in cable-casting systems. However, the standard practice in such systems is to use suppressed-carrier quadrature amplitude modulation (QAM) signals rather than VSB signals. This presents television receiver designers with the challenge of designing receivers that are capable of receiving either type of transmission and of automatically selecting suitable receiving apparatus for the type of transmission currently being received. Such receivers are denominated "QAM/VSB digital television receivers" in this specification and are sometimes referred to as "VSB/QAM digital television receivers". The design of QAM/VSB DTV receivers with intermediate-frequency (IF) amplifiers used in common for both QAM and VSB signals has been described by C. B. Patel and the inventor in their U.S. Pat. No. 5,506,636 issued Apr. 9, 1996, entitled HDTV SIGNAL RECEIVER WITH IMAGINARY-SAMPLE-PRESENCE DETECTOR FOR QAM/VSB MODE SELECTION, and incorporated herein by reference. This type of QAM/VSB DTV receiver is also described by C. B. Patel and the inventor in U.S. patent application Ser. No. 08/266,753 filed Jun. 28, 1994 and entitled RADIO RECEIVER FOR RECEIVING BOTH VSB AND QAM DIGITAL HDTV SIGNALS, U.S. Pat. No. 5,715,012 issued Feb. 3, 1998 and entitled RADIO RECEIVERS FOR RECEIVING BOTH VSB AND QAM DIGITAL HDTV SIGNALS, and in U.S. patent application Ser. No. 08/773,949 filed Dec. 26, 1996 and entitled RADIO RECEIVERS FOR RECEIVING BOTH VSB AND QAM DIGITAL HDTV SIGNALS. U.S. Pat. Nos. 5,506,636 and 5,715,012 and U.S. patent application Ser. No. 08/266,753 were written presuming that the carrier frequency of a VSB DTV signal would be 625 kHz above lowest channel frequency, as earlier proposed by a subcommittee of the Advanced Television Systems Committee. This specification presumes that the carrier frequency of a VSB DTV signal is nominally 310 kHz above lowest channel frequency, as specified in Annex A of the Digital Television Standard published Sep. 16, 1995.
The QAM/VSB DTV receivers described in U.S. Pat. No. 5,506,636 are subject to a problem of being locked out of VSB reception at times when VSB rather than QAM signals are being received. The inventor traces this problem, which sometimes occurs, to arising because one of the later local oscillators in each of these plural-conversion receivers receives automatic-frequency-and-phase-control (AFPC) signal as selected from one of two sources, depending on whether the DTV signal being currently received is QAM or VSB in nature. In the QAM/VSB DTV receiver described in U.S. Pat. No. 5,506,636, the selection of AFPC signal is controlled by an imaginary sample presence detector responsive to the circuitry used for synchrodyning a possible VSB signal to baseband. In order for the imaginary sample presence detector to operate satisfactorily, however, the circuitry used for synchrodyning a possible VSB signal to baseband has to be properly synchronized with regard to the VSB pilot carrier. Unless this state of proper synchronization is in existence, imaginary samples will occur. Responsive to the occurrence of these imaginary samples, the imaginary sample presence detector will condition the QAM/VSB DTV receiver for QAM reception. The circuitry used for synchrodyning a possible QAM signal to baseband will be referred to for providing AFPC signal for the controlled later local oscillator, rather than the circuitry used for synchrodyning a possible VSB signal to baseband, so proper synchronization with regard to the VSB pilot carrier is not forced. Proper synchronization can occur accidentally, in which case, the imaginary sample presence detector will condition the QAM/VSB DTV receiver for VSB reception. Slippage of phase between the VSB pilot carrier and the carrier generated for synchrodyning the VSB signal to baseband makes such accident likely. But sometimes there is no substantial slippage of phase between the VSB pilot carrier and the carrier generated for synchrodyning the VSB signal to baseband, and the phase persists in being incorrect. Under such conditions the lock-out from VSB reception mode occurs.
A similar problem of being locked out of VSB reception at times when VSB rather than QAM signals are being received is sometimes observed in the QAM/VSB DTV receivers described in U.S. Pat. No. 5,715,012 and in U.S. patent applications Ser. Nos. 08/266,753 and 08/773,949, in which receivers the selection of AFPC signal is controlled by a VSB pilot carrier presence detector responsive to the circuitry used for synchrodyning a possible VSB signal to baseband. The sensitivity to the difference in phase between the VSB pilot carrier and the carrier generated for synchrodyning the VSB signal to baseband tends to be less critical for the VSB pilot carrier presence detector to provide indication that a VSB pilot carrier is being detected than for the imaginary sample presence detector to indicate the non-occurrence of imaginary samples. Nevertheless, if there is no substantial slippage of phase between the VSB pilot carrier and the carrier generated for synchrodyning the VSB signal to baseband, and the phase persists in being 90.degree. away from correct synchronization, the lock-out from VSB reception mode occurs.
A primary objective of the inventor was to prevent any lock-out from VSB reception mode in a QAM/VSB digital television receiver using bandpass trackers for QAM reception and for VSB reception.